The impact categories that will be considered as part of the overall LCA were selected as part of the initial goal and scope definition phase. To guide the LCI data collection process and required reconsideration for an LCIA, impacts are defined as the consequences due to the input and output streams of a system on human health, on plants and animals (i.e. ecological health), or on the future availability of natural resources (i.e. resource depletion). Table 6.1 shows some of the more commonly used impact categories.
Step 2: Classification
For LCI items that contribute to only one impact category, the classification procedure is a straightforward assignment. For example, carbon dioxide emissions can be replaced in the global warming category.
For LCI items that contribute to two or more different impact categories, a rule must be established for classification. There are two ways of assigning LCI results to multiple impact categories (ISO 1998a):
- Allocate a representative portion of the LCI results to the impact categories to which they contribute. This is typically allowed in cases when the effects are dependent on each other.
- Assign all LCI results to all impact categories to which they contribute. This is typically allowed when the effects are independent of each other.
For example, since one molecule of sulfur dioxide (SO2) can stay at ground level or travel up into the atmosphere, it has the potential to affect either human health or acidification (but not both at the same time). Therefore, SO2 emissions typically are divided between those two impact categories (e.g. 50% allocated to human health and 50% allocated to acidification). On the other hand, since nitrogen dioxide (NO2) could potentially affect both ground level ozone formation and acidification simultaneously, the entire quantity of NO2 would be allocated to both impact categories (e.g. 100% to ground level ozone and 100% to acidification). The allocation procedure must be clearly documented.
Step 3: Characterization
Impact characterization uses science‐based conversion factors, called characterization factors, to convert and combine the LCI results into representative indicators of impacts to human and ecological health. Characterization factors also are commonly referred to as equivalency factors. Characterization factors translate different LCI inputs – for example, the toxicity data for lead, chromium, and zinc – into directly comparable impact indicators. With such data in hand, estimates of the relative terrestrial toxicity of these metals could be made.
Table 6.1 Commonly used life cycle impact categories.
| Impact category | Scale | Relevant LCI data (i.e. classification) | Common characterization factor | Description of characterization factor |
| Global warming | Global | Carbon dioxide (CO2) Nitrogen dioxide (NO2) Methane (CH4) Chlorofluorocarbons (CFCs) Hydrochlorofluorocarbons (HCFCs) Methyl bromide (CH3Br) | Global warming potential | Converts LCI data to carbon dioxide (CO2) equivalents Note: global warming potentials can be 50, 100, or 500 year potentials |
| Stratospheric ozone depletion | Global | Chlorofluorocarbons (CFCs) Hydrochlorofluorocarbons (HCFCs) Halons Methyl bromide (CH3Br) | Ozone depleting potential | Converts LCI data to trichlorofluoromethane (CFC‐11) equivalents |
| Acidification | Regional local | Sulfur oxides (SOx) Nitrogen oxides (NO) Hydrochloric acid (HCl) Hydrofluoric acid (HF) Ammonia (NH4) | Acidification potential | Converts LCI data to hydrogen (H+) ion equivalent |
| Eutrophication | Local | Phosphate (PO4) Nitrogen oxide (NO) Nitrogen dioxide (NO2) Nitrates Ammonia (NH4) | Eutrophication potential | Converts LCI data to phosphate (PO4) equivalents |
| Photochemical smog | Local | Non‐methane hydrocarbon (NMHC) | Photochemical oxidant creation potential | Converts LCI data to ethane (C2H6) equivalents |
| Terrestrial toxicity | Local | Toxic chemicals with a reported lethal concentration to rodents | LC50 | Convert LC50 data to equivalents |
| Aquatic toxicity | Local | Toxic chemicals with a reported lethal concentration to fish | LC50 | Convert LC50 data to equivalents |
| Human health | Global, regional, local | Total releases to air, water, and soil | LC50 | Convert LC50 data to equivalents |
| Resource depletion | Global, regional, local | Quantity of minerals used Quantity of fossil fuels used | Resource depletion potential | Converts LCI data to a ratio of quantity resource used versus quantity of resource left in reserve |
| Land use | Global, regional, local | Quantity disposed of in a landfill | Solid waste | Converts mass of solid waste into volume using an estimated density |
The impact categories listed in Table 6.1 have many possible endpoints, including the following:
| Global impact | |
| Global warming | Polar melt, soil moisture loss, longer seasons, forest loss/change, and change in wind and ocean patterns |
| Ozone depletion | Increased ultraviolet radiation |
| Resource depletion | Decreased resources for future generations |
| Regional impacts | |
| Photochemical smog | “Smog,” decreased visibility, eye irritation, respiratory tract and lung irritation, and vegetation damage |
| Acidification | Building corrosion, water body acidification, vegetation effects, and soil effects |
| Local impacts | |
| Human health | Increased morbidity and mortality |
| Terrestrial toxicity | Decreased production and biodiversity and decreased wildlife for hunting or viewing |
| Aquatic toxicity | Decreased aquatic plant and insect production and biodiversity and decreased commercial or recreational fishing |
| Land use | Loss of terrestrial habitat for wildlife and decreased landfill space |
Impact indicators are typically characterized using the following equation:
For example, all GHGs can be expressed in terms of carbon dioxide equivalents by multiplying the relevant LCI results by a CO2 characterization factor and then combining the resulting impact indicators to provide an overall indicator of global warming potential.
The Intergovernmental Panel on Climate Change provides conversion factors for a number of industrial pollutants. In the following example, the global warming impacts of different amounts of chloroform (CHCl3) and methane (CH4) are characterized. The value of the conversion factor, called GWP, for global warming potential, is 9 for CHCl3 and 21 for CH4.
Thus, we write:
Use of the conversion factors shows that 10 lb of methane has a larger impact on global warming than 20 lb of chloroform.
The key to impact characterization is using the appropriate characterization factor. For some impact categories, such as global warming and ozone depletion, there is a consensus on acceptable characterization factors. For other impact categories, such as resource depletion, a consensus is still being developed. Table 6.1 includes descriptions of possible characterization factors for some of the commonly used life cycle impact categories. A properly referenced LCIA will document the source of each characterization factor to ensure that they are relevant to the goal and scope of the study. For example, many characterization factors based on studies conducted in Europe cannot be applied to American data unless it can be verified that they are appropriate to conditions in the United States.
Step 4: Normalization
Normalization is an LCIA tool used to express impact indicator data in a way that can be compared among impact categories. In this procedure, the indicator results are divided by a reference value selected for the purpose. Reference value may be chosen from among numerous methods.
The following are representatives:
The total emissions or resource use for a given area that may be global, regional, or local.
The total emissions or resource use for a given area on a per capita basis.
The ratio of one alternative to another (i.e. the baseline).
The highest value among all options.
The goal and scope of the LCA may influence the choice of an appropriate reference value. Note that normalized data can only be compared within an impact category. For example, the effects of acidification cannot be directly compared with those of aquatic toxicity because the characterization factors were calculated using different scientific methods.
Step 5: Grouping
Grouping assigns impact categories into one or more sets to facilitate the interpretation of the results into specific areas of concern. Typically grouping involves sorting or ranking indicators. The ISO (1998a) lists two possible ways to group LCIA data:
- Sorting indicators by characteristics such as emissions (e.g. air and water emissions) or location (e.g. local, regional, or global)
- Sorting indicators by a system of ranking based on value choices, such as high, low, or medium priority.
Step 6: Weighting
Weighting (also referred to as valuation) assigns relative values to the different impact categories based on their perceived importance or relevance. Weighting is important because the impact categories should also reflect study goals and stakeholder values. But since weighting is not a scientific process, its methodology must be clearly explained and documented. The weighting stage is the least developed of the impact assessment steps and also is the one most likely to be challenged for integrity. In general, weighting includes the following activities:
Identifying the underlying values of stakeholders
Determining weights to place on impacts
Applying weights to impact indicators
Weighted data should not be combined across impact categories unless the weighting procedure is explicitly documented. The unweighted data should be shown together with the weighted results to ensure a clear understanding of the assigned weights.
In some cases, the impact assessment results are so straightforward that a decision can be made without the weighting step. For example, when the best‐performing alternative is significantly and meaningfully better than the others in at least one impact category and equal to the alternatives in the remaining impact categories, then one alternative is clearly better.
Several issues make weighting a challenge. The first issue is subjectivity. According to ISO 14042, any judgment of preferability is a subjective statement of the relative importance of one impact category over another (ISO 1998a). Additionally, these value judgments may change with location or time of year. For example, a resident of Los Angeles may place more importance on the values for photochemical smog than someone in Cheyenne, Wyoming. The second issue is derived from the first: how should users fairly and consistently make decisions based on environmental preferability, given the subjective nature of weighting? Trying to develop a truly objective (or universally agreeable) set of weights or weighting methods is not feasible. However, several approaches to weighting do exist and are used successfully for decision making.
Step 7: Evaluate and Document the LCIA Results
Now that the impact potential for each selected category has been calculated, the accuracy of the results must be verified. Documentation of the results of the LCIA entails thoroughly describing the methodology used in the analysis, defining the systems analyzed and the boundaries that were set, and setting forth all assumptions on which the inventory analysis was passed.
The LCIA, like all other assessment tools, has inherent limitations, including the following:
- Lack of spatial resolution (e.g. a 4000 gal ammonia release is worse in a small stream than in a large river)
- Lack of temporal resolution (e.g. a 5 T release of particulate matter during a one month period is worse than the same release spread through the whole year)
- Inventory speciation (e.g. broad inventory listing such as “VOC” or “metals” do not provide enough information to accurately assess environmental impacts)
- Threshold and nonthreshold impact (e.g. 10 T of contamination is not necessarily 10 times worse than 1 T of contamination)
The selection of more complex or site‐specific impact models can help reduce the limitations of the impact assessment’s accuracy. It is important to document these limitations and to include a comprehensive description of the LCIA methodology, as well as, a discussion of the underlying assumptions, value choices, and known uncertainties in the impact models with the numerical results of the LCIA to be used in interpreting the results of the LCA.
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